This invention is utilized at present as part of an infrared aircraft detection system. This system has 28 IR detectors arranged in a vertical line and located in a glass dewar. These detectors have to be cooled to 77.degree. K. by liquid nitrogen LN.sub.2 for proper operation. The detector-dewar assembly is located in the focal plane of a Schmidt optical system. This optical unit is rotated at a 1 revolution/second rate and becomes the scanning head of the infrared aircraft detection system. During cooling of the IR detectors and normal operation of the infrared aircraft detection system, the detector-dewar assembly is not visible to the operator. Damage to the glass dewar can occur if the LN.sub.2 chamber is overfilled and improper operation of the system will occur if the LN.sub.2 level becomes too low and the IR detectors start to warm. The Liquid Nitrogen Indicator was designed to indicate the level of LN.sub.2 in the dewar and prevent damage or improper operation of the infrared aircraft detection system.